It is conventional that catalysts, in particular NOx storage catalysts, are used to purify exhaust gases of combustion engines. The combustion engine is operated in a lean mode, in which the lambda value is greater than 1, i.e. there is an excess of oxygen in relation to the amount of fuel in the air-fuel mixture. In this operating mode, the proportion of environmentally harmful exhaust-gas components formed, such as carbon monoxide CO and incompletely burned hydrocarbons HC, is relatively small, and, due to the excess oxygen, they can be completely converted into compounds that are environmentally less relevant. On the other hand, the relatively large amounts of nitrogen oxides NOx formed in the lean mode cannot be completely reduced and are stored in the NOx storage catalyst in the form of nitrates. The NOx absorber is regenerated in regular intervals, in which the combustion engine is operated in a lean mode at λ≦1, and the reducing agents CO, HC, and H2 are formed to a sufficient extent, so that the stored nitrogen oxides can be quantitatively converted to nitrogen. In the rich mode, the release of nitrogen oxides from the NOx storage catalyst is aided by high temperatures at the catalyst.
In addition to the described storage of NOx, the storage of SOx in the form of sulfates also occurs in the NOx absorber in the lean mode. The absorption of SOx reduces the storage capacity of the absorber and the catalytically active surface area of the catalyst. Furthermore, the formation of sulfate particles can also bring about corrosive processes on the surface of the catalyst and cause irreversible damage to the NOx storage catalyst.
It is conventional to periodically carry out desulfurization processes, which include supplying rich exhaust gas, i.e. exhaust gas having a λ≦1, to the NOx storage catalyst, and setting a minimum temperature of approximately 600° C. that exceeds the NOx desorption temperature.
According to Published German Patent Application 198 358 08, the desulfurization is preferably not carried out in a constant rich mode of the combustion engine, at a rich lambda value, but rather by alternatingly supplying the NOx storage catalyst with rich and lean exhaust gas. In this manner, the release of poisonous and malodorous hydrogen sulfide H2S, whose formation may be kinetically inhibited in favor of the formation of sulfur dioxide SO2, can be suppressed almost completely.
The need for desulfurization may be determined, e.g. using NOx sensors, by detecting decreasing NOx storage activity or an NOx breakthrough in the lean exhaust gas in so doing, a fall in the NOx storage activity may be detected by comparing the measured NOx flow rate to a measured or modeled NOx flow-rate characteristic of a regenerated NOx storage catalyst. For lack of suitable sulfur sensors, sulfur contamination may presently be inferred from falling NOx activity, but not on the basis of a direct sulfur measurement.
As may be the case with determining the necessity for desulfurization, the success of a desulfurization process may also be determined on the basis of the NOx concentrations in front of and behind the catalyst, by merely detecting a recovery of the NOx activity. In this case, one also deduces that a residual amount of sulfur remains, by comparing the measured NOx flow rate to the state of a regenerated NOx storage catalyst. In this method it is not possible to check the progress during the desulfurization itself, but rather its success may be assessed after the desulfurization is finished. Since, in order to accomplish this, the NOx storage catalyst must first be re-cooled to the working temperature of approximately 200° C. to 500° C. and, when desulfurization is unsuccessful, reheated to the desulfurization temperature of greater than 600° C., this method is associated with higher fuel consumption. On the other hand, excessively long desulfurization procedures may thermally damage the NOx storage catalyst.